1. Field of the Invention
This invention relates to a display device for displaying images, and more particularly to a liquid crystal display device for generating and displaying images having sufficient contrast to be easily seen in bright daylight.
2. Description of the Related Art
A persistent problem in the art of electronically generating and displaying images is generating an image with sufficient contrast between light and dark areas to permit the features of the image to be easily discerned in bright sunlight and over a wide temperature range. The principal reason that many display technologies, including liquid crystal displays (LCDs) and cathode ray tubes (CRTs), are difficult to read in bright ambient light conditions is that they reflect a significant amount of incident ambient light, which essentially masks any emissive or transmissive light from the display. For example, the phosphors on CRTs reflect about 70% to 80% of incident light. The phosphor luminance forming displayed information simply gets overcome by the reflected luminance, rendering the display illegible in direct sunlight. Similarly, active matrix LCD (AMLCD) panels reflect about 50% of incident light. overcome by the reflected luminance, rendering the display illegible in direct sunlight. Similarly, active matrix LCD (AMLCD) panels reflect about 50% of incident light.
Sunlight is accepted in industry to be 10,000 foot candles of incident illumination with a spectral energy distribution profile that favors the blue wavelengths of light. It is also accepted in industry that contrast values of 5:1 or higher are necessary in high ambient lighting conditions if the display is to be daylight readable. This means an emissive display with high background reflectance, such as a CRT, would need to emit on the order of 35,000 fL-40,000 fL to be legible in direct sunlight. Considering that CRT displays that produce 200 fL-300 fL of luminance at their face are considered exceptional, achieving such a high level luminance in an emissive panel is extremely difficult.
In addition to such problems with emissive displays, it would be desirable for a number of reasons, such as power consumption and display size, to be able to use LCDs (sometimes referred to as light valve displays) as outdoor displays. Possible uses would be bank ATM machines and information kiosks. However, to date, such displays have not been daylight readable.
A typical LCD assembly consists of a liquid crystal cell and two linear polarizers. A first linear polarizer is disposed on the front surface of the liquid crystal cell. A second linear polarizer is disposed on the back of the liquid crystal cell. The polarizer can be envisioned as a fine parallel line grating having parallel lines spaced equidistant apart. For the linear polarizer to provide the effect needed for an LCD, the width of the lines and spaces of the polarizer must be approximately the size of the wavelength of light the polarizer is intended to selectively pass. The E-vector of light provided by a typical light source used with an LCD is generally completely random in orientation. However, only incident light waves having E-vectors aligned with the grating of the polarizer pass through virtually unaltered. The dark body of the polarizer in a liquid crystal display absorbs light that does not have aligned E-vectors, whether that light is incident on the front of the LCD or source from the rear. A good quality linear polarizer will pass 40$ to 45% of randomly aligned incident light (50% is the theoretical limit for linear polarizers).
The light valve resident between the polarizers is the liquid crystal cell. Liquid crystal materials are typically long chain-like molecules that have the unique property of being able to rotate from a first axis to a second axis of orientation when a voltage is applied across the cell gap. The degree of rotation is principally controlled by the magnitude of the voltage across the cell. The cell, in reference to the polarizers, can be thought of as a shutter. When the cell is turned on, it allows light from the rear illumination source (backlight) to pass completely through the assembly. When the cell is turned off, the display is dark. For color LCD displays, color filters are placed over discrete cell locations in a pixel pattern or mosaic) one each red, green, and blue sub-pixel making a white color dot) and the emission spectra in the backlight is matched to the color filters to make a color display.
LCD transmissive displays have inherently higher contrast in high ambient lighting environments than emissive displays because of the dark, nearly black, background color of the linear polarizer. The dark body of the polarizer in a liquid crystal display absorbs most light, whether incident on the front or sourced from the rear, that does not have a specific E-vector. However, some background reflectance of ambient light still occurs, particularly with Almonds. To meet the 5:1 contrast discussed earlier and overcome the effects of the background reflectance on the front surface due to sunlight, the LCD assembly would need to provide about 1,000 fL of luminance at its front face. While this is a lot less than emissive displays, it is still a very bright display (i.e., backlight) and beyond practical limitations imposed on such systems, such as the amount of heat which may be generated in a very high power system. For reference, typical notebook computer displays produce 20 fL-40 fL at the face of the display. State of the art military LCD displays generate as much as 200 fL at the display face.
Another problem with using conventional LCD panels in direct sunlight is due to heat. Typically, the majority of light incident on an LCD panel is absorbed. Light energy absorbed in the polarizer is translated into heat. Light absorption is a desirable optical characteristic, but such absorption increases the temperature of the liquid crystal cell, and thus limits the useful ambient temperature range of the displays. Liquid crystal displays can only perform optimally over a narrow band of temperatures (typical about 0.degree. C. to 40.degree. C.). At temperatures below 0.degree. C., the liquid crystal material loses its mobility and cannot readily react to a voltage applied across the cell gap. At temperatures above 40.degree. C., referred to as the "clearing temperature", a condition known as "clearing" occurs in which all cells move to an open state. Accordingly, information content is lost because there is no discernible contrast in the display.
The clearing temperature of a light crystal cell can be reached in at least two ways. First, the ambient air temperature can directly raise the temperature of the liquid crystal material. Second, the temperature of the liquid crystal material can be elevated by external thermal loading, such as from direct sunlight, even if the ambient temperature is below the clearing temperature for the material. External thermal loading from direct sunlight is more particularly known as solar loading. Accordingly, because an LCD can clear due to solar loading even when the ambient air temperature is well below the clearing temperature, solar loading imposes limitations on where and how LCDs can be used.
Therefore, there is currently a need for an LCD which is easily readable in daylight and which is usable over a wide temperature range in direct sunlight.